Asbestos has been used as a reinforcing fibre for cement which is excellent in such qualities as mechanical strength and is inexpensive. However, various kinds of inorganic fibres and synthetic fibres have been proposed which can be substituted for asbestos in reinforcing cement. This is advantageous since asbestos has caused many environmental problems.
For example, the use of glass fibres, polyester fibres, polypropylene fibres, aromatic polyamide, and acrylic fibres were disclosed in Japnese Patent Laid Open Sho (Tokkaisho)No.49-98424 (1974), No.49-104917 (1974), No.49-104918 (1974), No.61-86452 (1986), No.62-171952 (1987). In general, the structure of cement articles, which are shaped by conventional methods such as wet paper forming, extrusion or casting with the reinforcing fibres abovementioned, are made relatively dense by high-pressure pressing or cured under various kinds of conditions in order to improve the strength of the cement articles. Curing can be done naturally, by steam, or by an autoclave. Natural curing requires a long curing time, longer than fourteen days. On the other hand, autoclave curing, which is conducted at a high temperature, hotter than 140.degree. C., is advantageous in that curing lasts only twelve hours to eighteen hours normally.
Under alkaline conditions reinforcing fibres such as polyester fibres, vinyl fibres, polyamide fibres and acrylic fibres undergo chemical changes and become brittle when cured at the high temperatures abovementioned.
In addition, even alkaline-resistant glass fibres can become brittle when they are cured at the high temperatures abovementioned. Fibres which can withstand curing at high temperatures are heat-resistant alkaline polyolefin fibres such as poly 4-methyl pentene-1.
However, conventional polypropylene fibres are generally crystalline polypropylene having 96&lt;HI&lt;97 and 93&lt;IPF&lt;94 and normally having 5-6 g/d strength, HI being insoluble content of boiling normal-heptane in wt % and IPF being the isotactic pentad fraction in mol %. In addition, it is generally known that dry state fibre drawing which is conducted at about 150.degree. C. with polypropylene having relatively low value of Q, about 4, is carried out in order to improve the drawing property of polypropylene. However, the polypropylene, which is drawn by the dry drawing practice, has strength of 7.5 g/d at maximum. As polypropylene is inferior in strength to other fibres, it is generally limited to being used as a reinforcing fibre for cement.
Polypropylene fibre which is made of a highly crystalline polypropylene fibre is disclosed in Patent Laid Open Sho (Tokkaisho) No.60-59113(1985), No.62-41331(1987). A problem exists in that the polypropylene fibres abovementioned have a low fibre breakage strength. In addition, in the past, highly crystalline polypropylene was mainly used in injection molding. And the polymer which possessed a wide molecular weight distribution, Q having a value of grater than 6, was put on the market in order to prevent heat contraction. The resin had a high value of Q and also had the same value of drawing orientation as conventional polypropylene. However, conventional polypropylene has been inferior to other synthetic fibres in strength. Recently, other synthetic fibres have been improved in strength remarkably. In comparison with the improved synthetic fibres abovementioned, the strength of conventional polypropylene has declined relatively. Improvement of strength of polypropylene fibres is required in the field of reinforcing short fibres for cement which primarily requires the strength. However, at present, the reinforcing short fibres for cement possessing the required property have not been obtained.